1. Field of the Invention
The present invention relates to a projection type image display apparatus such as a projector, etc.
2. Description of the Prior Art
A conventional projection type image display apparatus normally uses a liquid crystal panel or micro mirror device as a light modulation element for passive switching, projects a light pattern selected by controlling transmission, shielding or deflection of light onto a screen, and thereby displays the image on the screen.
When a liquid crystal panel or micro mirror device is used as a light modulation element, it is assumed that shielded light is regarded as unnecessary energy and absorbed by a polarization element or light absorption medium and eliminated. In the case of a liquid crystal, unnecessary illumination light exists due to light transmittance, aperture. efficiency of each pixel and accuracy of polarization control, and in the case of a micro mirror device, it is difficult to effectively use the pupil of an axially symmetric optical system from the standpoints of aperture efficiency of each pixel, numerical aperture of projection lenses due to oblique-incidence illumination and numerical aperture of an illumination system.
Therefore, in order to make a displayed image brighter, a metal halide or high-pressure mercury lamp is used as a light source, but there are additional problems that a high voltage needs to be used as a light source voltage and the light source produces high heat.
As a method for fundamentally solving such low efficiency of energy usage, there are proposals disclosed in Japanese Patent Laid-Open No. 1999-67448 and Japanese Patent Laid-Open No. 2000-66301. In the above-described two proposals, a luminescent panel (hereinafter referred to as “organic EL panel”) on which organic electroluminescence elements (hereinafter referred to as “organic EL (electroluminescence) elements”) are arranged in matrix form is used, light emission of each organic EL element of this luminescent panel is driven based on an image signal and light from each organic EL element is projected onto a display target object through a projection optical system. Since the organic EL element is a self-luminescent element, there is no need for any separate illumination light source and the organic EL panel emits pattern light including image information, therefore there is no need for a transmission type liquid crystal panel as a modulation element. Therefore, it is possible to use emitted light for displays effectively.
The organic EL element is therefore expected to be able to easily display images with lower power and high brightness without generating unnecessary light energy. Furthermore, since the organic EL panel alone can modulate and output images, the apparatus configuration is simple and it is easy to miniaturize the apparatus and reduce its weight.
However, when the organic EL panel is used as an image modulation light source for a projection type image display apparatus that enlarges an image, and projects and displays the image on an object such as a screen, it is necessary to increase the amount of light emitted from the organic EL element to a great extent compared to a case where the organic EL panel is used as a direct-view type display. That is, it is necessary to increase the amount of light emitted by increasing power energy directed to pixels arranged in the organic EL panel.
However, there is a problem that when light is emitted from the organic EL element at high brightness, the luminance efficiency of the amount of emitted light with respect to the power input reduces.
This is because increasing the amount of charge carriers injected into the luminescent layer, that is, the amount of current requires an applied voltage to be increased, and increasing the voltage is equivalent to increasing the gradient of the electric field applied to the luminescent layer. In contrast to an electroluminescent emission phenomenon in which electrons and holes remain in the luminescent layer area and excitons are formed when this gradient of the electric field increases and photons are emitted by recombination, charge carriers do not remain in the luminescent layer area, a leak current that penetrates the luminescent layer increases whereby the luminescent efficiency of the amount of light emitted with respect to the power input decreases.
To prevent charge carriers from brimming from the luminescent layer and improve the luminescent efficiency, development toward the adoption of a double-hetero potential structure is underway. However, the present situation is that the height of the potential barrier on the LUMO side of the electron blocking layer and the height of the potential barrier on the HOMO side of the hole blocking layer depend on component materials and this technology does not reach a level of sufficiently functioning with respect to the electric field gradient applied to the luminescent layer.
For this reason, rather than projecting light from the organic EL panel consisting of luminescent pixels of three primary colors arranged in a repetitive matrix form, a configuration using three organic EL panels that emit three primary colors separately, combining three color light using a wavelength-combining element such as a prism provided with a dichroic waveband separating film and projecting the combined light will reduce the amount of power required for one pixel and improve the luminescent efficiency of the amount of light emitted with respect to power input.
However, because the organic EL element generally emits non-polarized light, or due to isotropy of luminescent directivity, a separated wavelength shift characteristic depending on the angle of incidence of light upon the dichroic waveband separating film and a difference in the separated wavelength of the dichroic waveband separating film between S-polarization and P-polarization, this configuration has difficulty in combining light emitted from the organic EL elements without loss. That is, even if light from three organic EL panels that emit three color light respectively is combined, the light utilization efficiency is not equivalent to a simple sum of those.
Thus, using a layered organic EL panel which laminates three organic EL luminescent layers that emit three primary color light can reduce the amount of power input to each organic EL luminescent layer and improve the efficiency of the amount of light emitted (amount of light projected onto an object such as a screen) with respect to the amount of power input without reducing the efficiency of combining three color light.
However, when a layered organic EL panel is used, due to a shift of positions in the thickness direction of three organic EL luminescent layers, if light is projected onto an object such as a screen using a projection lens, the image forming position varies depending on the color, which causes a color-dependent blur in the image. This image blur becomes more noticeable as the projection magnification increases.
On the other hand, when a liquid crystal panel or micro mirror device is used as a light modulation element, a maximum output brightness distribution of each pixel modulated by the modulation panel depends on an illumination distribution of an illumination system that illuminates the light modulation element by transmission or reflection. Therefore, the illumination distribution is designed according to the configuration of the illumination optical system. When a gas discharge lamp using a paraboloid or spheroid mirror is used, the light beam shows an atypical Gaussian distribution and it is a general practice to convert this distribution to a uniform illumination distribution using an integrated optical system and it is difficult to use a method of providing an intentional illumination distribution other than the uniform distribution.
To solve this problem, Japanese Patent Laid-Open No. 2000-75406 proposes a method of providing a brightness distribution for a panel-shaped surface luminescent type light source that illuminates the light modulation panel to compensate for the reduction in the amount of light transmitted outside the optical axis of a projection lens in order to intentionally make illumination of the screen which is a projection target object uniform.
As the panel-shaped surface luminescent type light source, an electroluminescence type fluorescent luminous body light source or charge-injection type organic EL element is used. The above-described publication describes it as a light source that illuminates a light modulation element using a liquid crystal panel or micro mirror device, but similar effects are also attained even if the light source itself is a light modulation element.
However, the organic EL element has a problem with durability that the photoelectric conversion efficiency decreases gradually. This is because the organic luminescent material itself changes a chemical structure, repeats emissions of light energy according to the displacement of its potential energy and the organic luminescent layer is structured in such a way that a fluorescent or phosphorescent luminescent material, a material to disperse this material and a material to improve an electrical conductive characteristic are mixed, and therefore the probability that the organic luminescent material that changes its chemical structure and emits light will change to any forms other than a desired one does not become 0% in principle because the state distribution function is dispersed at a temperature equal to or higher than absolute zero.
The speed at which the luminescent material changes to any form other than the desired form is affected by the stability of the state of the material, combination with the binder environment medium material, intensity of the applied electric field and hydrolysis by a humidity environment, etc. A main factor for deterioration concerning the speed of chemical reaction is considered to be acceleration of deterioration by a self temperature rise parameter due to generation of thermal energy in the process of photoelectric conversion. This speed of deterioration becomes accelerated reaction almost in accordance with Arrhenius' reaction rate relational equation.
Therefore, if the system is driven to emit light at a low temperature with low power input, the speed of variation in the photoelectric conversion efficiency slows down and its life is extended, but when the power supplied is increased in order to increase brightness of light emissions, the photoelectric conversion efficiency decreases exponentially.
Therefore, assuming that a quality assurance period for a normal consumer product is one year, stable light emission for several thousands to several tens of thousands of hours is required. Attempting to provide clearer images of higher quality results in reduced stability in quality.
Furthermore, in a situation in which a projection type image display apparatus is used, an object such as a screen onto which an image is projected is placed in an illuminated environment in a room or place and the image is observed when an observer recognizes the diffused light from the object, and therefore it is a general practice to darken the illumination environment in which the object such as a screen is illuminated in order to enhance black-and-white contrast of the projected image. Movie projection is one of the typical examples.
On the other hand, when images for a presentation is displayed, illumination at hand is required to allow audience to refer to documents or take notes, and therefore the system may be used in a certain illumination environment. Thus, the operating environment of the projection type image display apparatus varies depending on the purpose.
Furthermore, while it is more important to display images more clearly in a bright operating environment, the image quality such as uniformity in the brightness of the image, clearness of contrast, or color reproducibility, etc., becomes more important.